1. Field of the Invention
The present invention relates generally to an auto focusing apparatus and method for a camera and, more particularly, to an auto focusing apparatus and method for a camera, which allows a camera module, which is mounted on a mobile phone, a smart phone, a personal digital assistant or the like, to acquire discrete cosine transform coefficients from a compression module, and to adjust the focus of a lens based on the acquired coefficients.
2. Description of the Related Art
Recently, with the rapid development of an information society, the development of a composite mobile communication terminal, in which a variety of functions are provided, is required in addition to a mobile communication terminal that merely transmits voice.
Accordingly, to meet the demands of the multimedia age, a composite mobile communication terminal, in which a function of transmitting and receiving images is combined with a function of transmitting and receiving voice, has been implemented.
Such a composite mobile communication terminal is exemplified by a camera phone, in which a digital camera function is implemented in a mobile communication terminal (so-called mobile phone) that is usually carried by a user.
When a user suddenly desires to take and keep a photograph of a certain scene or a subject required for business use, the camera phone allows the photograph to be taken and stored therein.
Furthermore, the camera phone allows the photographed image to be wirelessly transmitted through a mobile communication terminal to another mobile communication terminal, and the photographed image can be output onto the screen of a Personal Computer (PC) and stored in the PC when mobile communication terminal is connected to the PC.
A technology regarding a camera phone is disclosed in Korean Pat Appl. No. 99-60177, entitled “Photographed image transmission method for camera phone.”
Furthermore, recently, a mobile communication terminal (so-called Television (TV) phone) that is capable of receiving and outputting TV broadcasting programs, and a mobile communication terminal that is capable of downloading Internet information and displaying moving images when connected to the Internet have been developed. A next generation mobile communication terminal that allows all the functions of the mobile communication terminals to be performed is being developed.
The construction of a typical camera phone includes a camera module for photographing images, a transmission module for transmitting a user's voice and image, and a reception module for receiving the counterpart's voice and image.
The camera module includes a lens subsystem and an image processing subsystem.
The lens subsystem includes a lens unit composed of a zoom lens and a focus lens, an actuator for driving the zoom and focus lenses, and an actuator driver.
The image processing subsystem includes an image sensor and Image Signal processor (ISP) unit, and an auto focus Digital Signal Processor (DSP).
The lens subsystem focuses on a scene to be photographed, and allows light, which is incident on a specific region having a predetermined range from the scene, to come into contact with an image sensor.
The image sensor of the image processing subsystem includes photo-cells that accumulate charges as the light comes into contact with the image sensor during a specific absorption period, convert the accumulated charges into a digital value (pixel value), and output a resulting digital value.
The ISP unit of the image processing subsystem performs image processing, such as compression and scaling image enhancement, on acquired digital values corresponding to pixels, and transmits resulting values to a mobile phone main body.
In this case, the lens subsystem adjusts the focus of a lens to photograph a sharp image using an auto focus device similar to that utilized in a typical photo or digital camera without change. The method is briefly described below.
Generally, when the composition of a subject that is intended to be photographed is set and then a release button is operated, an auto focus device in a photographing apparatus, such a photo camera or a digital camera, takes a picture while automatically focusing on the subject.
The auto focus device employs an active method and a passive method.
The active method is a method of measuring a distance to a subject by emitting infrared rays or ultrasonic waves and sensing incident rays or waves that are reflected from the subject.
The passive method is a method of determining a distance to a subject by receiving light that is emitted from the subject through the lens unit under natural illumination without a separate emission unit that emits light, and using the light and darkness difference of the subject.
That is, the passive method performs an auto focus adjustment function in such a way as to detect a high band frequency signal, which is a component proportional to contrast obtained by passing the luminance signal of an image signal output from an image sensor through a high-band filter, for each frame, to move a focus lens in a direction that increases the contrast while comparing the obtained contrast with the contrast of a previous frame, and to stop the rotation of a focus lens actuator in a state of maximum contrast.
FIG. 1 is a block diagram showing the construction of a camera module in which a conventional auto focus adjustment apparatus is included. The camera module includes a lens unit 11, an image sensor and ISP unit 12, an auto focus DSP 14, an actuator driver 15, and an actuator 16.
The lens unit 11 includes a zoom lens and a focus lens. The zoom lens is a lens for magnifying an image, and the focus lens is a lens for focusing an image.
The image sensor employs a sensor using a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS). The ISP is a part for performing image processing, such as auto white balancing, auto exposure, and gamma correction, so that an image is converted to fit human vision.
The image sensor and ISP unit 12 converts image-processed data into a CCIR656 or CCIR601 format (YUV space), receives a master clock from a mobile phone host 13, and then outputs image data Y/Cb/Cr or R/G/B to the mobile phone host 13 along with a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync and a pixel clock Pixel_Clock.
The auto focus DSP 14 includes an Optical Detection Module (ODM) and a Central Processing Unit (CPU) for performing an auto focus algorithm based on the resulting values of the ODM.
In this case, the ODM is generally composed of a high band-pass digital filter, an integrator, and a window control block.
When the auto focus DSP 14 receives image data from the image sensor and ISP unit 12 and passes the received image data through the high band-pass digital filter, only the edge components of an image are extracted.
In this case, with respect to a window set region, a window control block receives the start and end positions of the window, and integrates the output values of the high band-pass digital filter inside the window in the integrator. Focus values obtained by the integration are used as reference data to adjust a focus in the camera module.
Generally, for a still image, the focus is adjusted by moving the lens unit 11. For the same image, when the image is in focus, a focus value becomes high. In contrast, when the image is out of focus, a focus value becomes low.
Referring to FIG. 2, when the same image is input to a camera, a low focus value is generated, as in regions “A” or “C,” when the image is out of focus, and a high focus value is generated, as in a region “B,” when the image is in focus. Meanwhile, for a complex image, the focus values in the region “B” are higher, and for a simple image, the focus values in the region “B” are lower. Generally, a camera is focused on the center of an image, and a window is placed on the basis of the center.
To find the maximal focus value of a screen, the actuator driver 15 causes the lens unit 11 to move by operating the actuator 16. The location where the focus value is maximal, as shown in FIG. 2, must be found by moving the lens unit 11.
The camera module determines whether to move the lens unit 11 forward or backward and controls the actuator driver 15 by executing an algorithm to find the maximal focus value in the CPU.
Meanwhile, in a mobile communication terminal, an auto focus adjustment device must be implemented using integrated circuits that require minimal cost and space. Ideally, an auto focus adjustment method is not be implemented using specific circuits that only automatically adjust a focus, but must be implemented using components that are also used for other purposes in an image apparatus.